Workers set up a model of the seafloor that they will submerge in a tank to simulate wave formation. / The Coriolis Facility

by Traci Watson, Special for USA TODAY

by Traci Watson, Special for USA TODAY

A 70-foot wave is a terrifying wall of water that only the best surfers can ride. But it's a midget compared with the colossal and mysterious waves that lurk under the ocean's surface.

These underwater waves, though seldom noticed, can rival skyscrapers in height and measure more than 100 miles wide. They can imperil submarines and disrupt operations on offshore oil platforms. They've been photographed by astronauts in orbit, and they've been cursed by bewildered sailors. Scientists are gaining fresh insights into these massive waves, including their potentially important role in climate change.

Underwater waves "are the lumbering giants of the ocean," says MIT oceanographer Thomas Peacock, who studies them. Such waves "roll around the planet. â?¦ But we see only little glimpses of them, because we live our lives above the surface."

These mammoth bulges of water, properly known as internal waves, owe their existence to the structure of the ocean, which is layered like a birthday cake. The upper layer - the water closest to the surface - is warmer and lighter, while the lower layer is colder and denser. When the tide pushes the layers over a ridge on the seafloor, a gargantuan wave is born at the transition from warmer to colder water, where the frosting between the layers would be. Winds from big storms also can generate internal waves.

These deep waves are the same shape as their whitecapped cousins on the sea surface. But a typical surface wave is 1 to 2 feet high, while a run-of-the-mill internal wave measures 15 to 30 feet out at sea, larger when it gets close to shore. Like the wind itself, the waves are both ubiquitous and, to the untutored eye, invisible.

"The whole ocean is filled with these internal waves," says Maarten Buijsman, a physical oceanographer at the University of New Orleans, but "it's really hard to see them if you're standing on the beach." That's because these behemoths create only tiny ridges of water on the ocean's surface. Someone who knows what to look for, though, may spot the alternating bands of smooth and rough water created by the meeting between internal waves and plain old surface waves.

Underwater waves move at a ponderous 6 to 7 mph, but their leisurely pace conceals their immense power. Submariners during World War II knew to avoid the Strait of Gibraltar, famed for its internal waves, says David Farmer, a physical oceanographer at the University of Rhode Island. In the 1980s, a Soviet submarine smashed into the bottom of a container ship, presumably because an internal wave tossed the sub as if it were a bath toy. Internal waves have threatened offshore oil-drilling rigs, and the stretches of calm water they create have bedeviled sailors.

Scientists have made the first measurements of internal waves breaking at a crucial spot in the Pacific. About 200 miles northeast of Samoa, a huge volume of seawater - equal to 35 Amazon rivers - barrels through a narrow underwater channel, then wends its way into the depths of the northern Pacific. Researchers led by University of Washington oceanographer Matthew Alford found that 800-foot-high internal waves act like a gargantuan mixer at the spot, churning together seawaters of different saltiness and temperature until they're thoroughly blended together.

The discovery provides insight into the source and strength of ocean turbulence, which helps drive the Earth's "circulation," the global movement of seawater that is crucial for understanding climate. The scientists reported their results last fall in Geophysical Research Letters.

Half an ocean away from the Samoan "nozzle," as Alford calls it, some of the world's biggest internal waves are born in the waters of the South China Sea between Taiwan and the Philippines. These giants among giants can measure more than 1,000 feet high, but the biggest collapse as soon as they're born. Only the "smaller" ones continue out to sea, and even these can be some 450 feet high, taller than the Statue of Liberty. To understand exactly what spawns these monsters, Peacock's team built a precise 16-foot-long model of the bottom of the South China Sea and immersed it in a rotating water tank almost as wide as a basketball court.

They recreated the tides by pushing the water in the tank to and fro with wedge-shaped paddles, while the rotation of the tank simulated the spinning of the Earth. The experiment, the largest ever in the laboratory to study internal waves, showed the researchers that the big waves are spawned by water sloshing over entire underwater ridges, not just one small spot on the ridge. Their study was published in November in Geophysical Research Letters.

More than pure science is at stake in such studies. These low-profile waves are "absolutely key for understanding climate change," Alford says. Internal waves influence ocean turbulence, which in turn affects the grand network of ocean currents that carries heat around the globe.

Recent research suggests that there will be more internal waves in the Arctic Ocean as the summer sea ice there shrinks because of climate change, Peacock says. That's likely to increase mixing of the sea, which could bring warm water from the bottom to the surface and lead to even more ice loss.